1. Field of the Invention
The present invention relates to a method for identifying a stylus on a touch panel and more particularly to a method for distinguishing correct sensed signals of touch objects on a touch panel from noises.
2. Description of the Related Art
When a touch panel is mounted on a liquid crystal module (LCM), noises are generated because of the scanning of the scan lines on the LCM and the noises are hereinafter called LCM noises impacting on positional precision of the touch panel in determining touched points on the touch panel. With reference to FIGS. 5A and 5C, capacitance variation values, dVstylus and dVfinger are equal to a base value subtracted by the capacitance values of true respective sensed signals of a stylus and a finger therefrom, respectively correspond to control ranges for sensing a stylus and fingers on a touch panel, and increase as a result of the LCM noises. If the capacitance variation values, dVstylus and dVfinger need to be correctly calculated, the capacitance variation values caused by the corresponding LCM noises must be deducted therefrom. To tackle the LCM noises, regular approaches preset a fixed noise threshold value and directly deducts the noise threshold value from the capacitance variation values of the sensed signals of stylus or finger so as to avoid the influence of the LCM noises.
However, with reference to FIG. 5B, as the true capacitance variation value of the LCM noise dVRnoise is not a constant, it's difficult to decide a fixed noise threshold value. If the noise threshold value is preset to be a relatively low constant, due to the fluctuating dVRnoise, it is easy for a sensed signal to be incorrectly identified as that of a stylus. On the contrary, true touched points of the stylus may be ignored. Since dVfinger is normally greater than dVstylus and dVnoise, the sensed signals for fingers are not easy to be incorrectly identified as those for other touch objects.
Sensing a stylus or fingers touching on a touch panel is commonplace nowadays. How to correctly identify the position touched by the stylus is definitely the name of the game.